As air pollution has worsened around urban centers, standards aimed at significantly cutting back nitrogen oxides, which are a cause of respiratory problems, and hydrocarbons, which are a cause of photochemical smog, have been established by the Central Environment Council of the Ministry of the Environment, as announced in 2004 on the basis of the Air Pollution Control Law. More specifically, exhaust gas restrictions on motorcycles and other two-wheeled vehicles are scheduled to be greatly tightened in 2006-2007 and beyond. As an example of the new standards, it is planned that as of 2006, scooters of 50 cc or smaller must emit no more than 0.5 g of hydrocarbons per kilometer of travel (a reduction of 75% over the current level), and no more than 0.15 g of nitrogen oxides (50% reduction). For motorcycles over 250 cc, it is planned that as of 2007, they must emit no more than 0.3 g of hydrocarbons (85% reduction) and no more than 0.15 g of nitrogen oxides (50% reduction).
Hydrocarbons are emitted as a result of incomplete combustion of gasoline, and their emission by two-wheeled vehicles is more than ten times that by passenger vehicles, accounting for some 20% of the total emissions of four- and two-wheeled vehicles combined. Given this situation, the use of the same catalytic converters in two-wheeled vehicles as those used in passenger vehicles has been studied, but existing catalytic converters would be disproportionately costly on inexpensive two-wheeled vehicles, and a less expensive exhaust gas purification catalyst needs to be developed.
In today's automobile catalytic converters, platinum, palladium, platinum/rhodium, palladium/rhodium, platinum/palladium/rhodium, and the like are supported on a monolithic carrier made from cordierite, and used as a three-way catalytic converter. Further, ceria, which is an oxygen-storing substance, has been used as an auxiliary catalyst in order to absorb fluctuations in the air-fuel ratio. The above noble metals have high catalytic activity, but they are costly, and moreover need to be used in large quantities, so they are recovered and reused.
An example of substances known and put to use in the past as active oxygen developing substances is a photocatalyst typified by titanium oxide. Electrons and holes are formed when light (UV rays) is absorbed by titanium oxide. Because the oxidative strength resulting from holes is greater than the reductive strength resulting from excited electrons with titanium oxide, adsorbed water on the catalyst surface is oxidized by the holes, producing hydroxy radicals (.OH). Meanwhile, a reaction proceeds in which the oxygen in the air is reduced, producing active oxygen (O2−). It is believed that active oxygen becomes water via hydrogen peroxide (H2O2) or the formation of a peroxide of an intermediate of the oxidation reaction. There are also cases in which active oxygen acts directly on carbon-carbon bonds and decomposes harmful organic substances.
Another substance known to develop active oxygen is a 12CaO.7Al2O3 compound that encloses active oxygen species (Japanese Laid-Open Patent Application No.2002-3218). This 12CaO.7Al2O3 compound is manufactured from a raw material mixture of calcium and aluminum in an atomic equivalent ratio of 12:14, which are subjected to solid phase reaction in a dry oxidative atmosphere controlled to an oxygen partial pressure of at least 104 Pa, and preferably at least 105 Pa, and a water vapor partial pressure of no more than 1 Pa, and at a high firing temperature of at least 1200° C., and preferably 1300° C. The active oxygen enclosed by a 12CaO.7Al2O3 compound manufactured in an atmosphere in which the oxygen partial pressure and water vapor partial pressure are strictly managed, which requires a large quantity of thermal energy, is O2− and/or O−.
However, when it came to this type of active oxygen developing substance, no inorganic compound that would include or occlude both a superoxide anion (O2−) and a peroxide anion (O22−) was known up to now, and all that was known was the production of a superoxide with a 12CaO.7Al2O3 compound or a photocatalyst such as the above-mentioned titania.
Furthermore, the release of volatile organic compounds and the like into the atmosphere today is causing serious environmental pollution. Combustion is one way that volatile organic compounds can be removed, but this requires high temperatures over 1000° C. Catalysts are utilized to allow combustion to occur at lower temperatures. The catalysts used for such applications are called combustion catalysts. In the past, oxides of cobalt, copper, manganese, chromium, and the like have been supported on porous alumina, allowing the combustion temperature to be lowered to between 300 and 600° C. (see, for example, (1) Y. M. Kang and B. Z. Wan, Appl. Cat. A, Vol. 114 (1994), p. 35, (2) R. S. Drago, K. Jurczyk, D. L. Singh, and V. Young, Appl. Cat. B8 (1996), p. 155, and (3) N. Watanabe, H, Yamashita, H. Miyadera, and S. Tominaga, Appl. Cat. B8 (1996), p. 405). Today, however, the development of a catalyst with higher activity than conventional catalysts is needed for the sake of energy conservation. Catalysts supporting noble metals such as platinum, palladium, and rhodium have high activity and are commonly used in automobile catalytic converters. Although these noble metals have high catalytic activity, they are also valuable and costly, and are used in large quantities, and therefore are recovered and reused. Hydrocarbons, which are environmental pollutants emitted from internal combustion engines, are emitted as a result of the incomplete combustion of gasoline, and their emission by two-wheeled vehicles is more than ten times that by passenger vehicles, accounting for some 20% of the total emissions of four- and two-wheeled vehicles combined. Given this situation, the use of the same catalytic converters in two-wheeled vehicles as those used in passenger vehicles has been studied, but existing catalytic converters would be disproportionately costly on inexpensive two-wheeled vehicles, and a less expensive exhaust gas purification catalyst that does not make use of noble metals needs to be developed in this field of technology.